We have used fluorescent probes, light scattering and circular dichroism to study the molecular interactions of compound 48/80. We have shown that pretreatment of rat peritoneal cells and mastocytoma cells by 48/80 increases the binding of both 1-anilinoaphathalene-8-sulfonate and Rhodamine-labeled 48/80 (R-48/80). The binding of 12-anthroyl-stearic acid was not affected. The emission maximum of R-48/80 did not change after binding to mastocytoma cells, whereas that of 12-anthroyl-stearic acid showed a blue shift indicating binding in a region of low polarity relative to the R-48/80. Fluorescence microscopy of mast cells labeled with R-48/80 showed it bound to the plasma membrane at low concentrations and to intracellular granules at high concentrations. There was no patching or capping. Light scattering by intact cells and cell membranes was increased by both 48/80 and heat-treatment. Protein depleted liposomes were not affected. Ultraviolet circular dichroism studies of mastocytoma cell membranes showed that 48/80 and heat-treatment however, did not significantly alter the ultrastructure of the membrane proteins. It is concluded that 48/80 binds to a receptor which is basically different from that of the fatty acids and which may be a protein.